1. Field of the Invention
The present invention relates to a thermoplastic resin composition and a process for producing the same. More particularly, it is concerned with a thermoplastic resin composition which has a good appearance and superior physical properties and also with a process for producing said thermoplastic resin composition in such a manner as to keep its quality stable
2. Description of the Related Art
Among composite resin materials whose matrix resin is a thermoplastic resin, fiber-reinforced thermoplastic resins are particularly important in many application areas because of their lightweight properties and superior mechanical strength.
In the case where the reinforcement is dispersed glass fibers or similar long fibers, the resulting fiber-reinforced thermoplastic resin has a drawback of being poor in external appearance and dimensional stability due to warpage and deformation, although it has at least improved mechanical strength. On the other hand, the one reinforced with short fibers such as wollastonite and milled fiber with a small aspect ratio is not satisfactory in the improvement of stiffness. For this reason, there has recently been proposed a process for producing a composite material having a high stiffness and good external appearance by the incorporation of extremely fine fibers with a high aspect ratio. (See Japanese Patent Laid-open No. 109846/1982.)
Unfortunately, the extremely fine fibers with a high aspect ratio have such a low bulk density of 0.02 to 0.1 that their stable production is difficult to achieve. Moreover the fibers do not disperse well into the matrix on account of their strong tendency toward agglomeration, which gives rise to the gelling on the surface of the molded article. Agglomeration can be eliminated only by mixing with a high shear force, which breaks fibers into short fibers.
On the other hand, there is disclosed a method for incorporating brittle inorganic fibers into a thermoplastic plastic resin without breaking. (See Japanese Patent Laid-open No. 17253/1984.) According to this method, mixing is accomplished by using an extruder having independent feed openings for individual raw materials. The first feed opening is located far from the extruding end, and the second one is located close to the extruding end. To perform mixing, a thermoplastic resin is fed through the first feed opening and then heated in the extruder. When the thermoplastic resin reaches the second feed opening, it is in a molten state. To this molten resin is added fibrous magnesium oxysulfate through the second feed opening. Thus the molten resin and the fibrous magnesium oxysulfate are mixed in the extruder.
This method, however, has the following disadvantages. The fibrous magnesium oxysulfate is so fine and has such a high aspect ratio that it easily gets entangled. The entangled fibers do not get loose completely but clog the extruder screen pack, disabling extrusion. Unevenly dispersed fibers cause the molded articles to look poor and adversely affect the performance of the molded articles. In addition, the fibrous magnesium oxysulfate is so bulky (with a bulk density lower than 0.1) that it cannot be fed into the extruder smoothly. Therefore, only a small amount of the fiber can be incorporated into the molten resin.
The present invention was completed to overcome the above-mentioned disadvantages involved in the prior art. Accordingly, it is an object of the present invention to provide a thermoplastic resin composition which can be formed into molded articles having a good external appearance and superior physical properties. It is another object of the present invention to provide a process for producing said thermoplastic resin composition in a stable manner.